1. Field of the Invention
The present invention relates to a working vehicle, and more specifically to a working vehicle which performs works at a low speed such as a motor grader.
2. Description of Related Art
There has been known the technique of improving a surface of a road or a ground by cutting or smoothing it to a flat state with a motor grader as one of a number of working vehicles (including earth-moving machines or construction machines). The land-improving work with the motor grader is performed mainly with a blade as a working machine, and high precision is required in this work, so that the vehicle's speed is kept low so that the works can be carried out carefully.
In the motor grader, the rear wheels are driven by a diesel engine via a torque converter and a transmission (with a tandem device), and when the motor grader is applied to the land-improving work, the revolution speed of the engine is dropped to a level near that at which the engine stops (the so-called engine-stopped state) and is driven at a low revolution speed of the engine. In the motor grader engine, it is possible like in other types of working vehicles to select either the low idling mode in which a revolution speed of the engine in the idling state is kept low or the high idling mode in which the revolution speed of the engine is kept high, and the land-improving work at the low revolution speed of the engine is inevitably performed in the low idling mode.
When a higher duty of the engine is required to respond to a larger ascending slope of a land to be improved or to a temporal increase of the land-cutting rate caused by local irregularities, the speed of the vehicle becomes further slower, and also a revolution speed of the engine drops with the engine stop easily occurring. When the event as described occurs, the operator operates an accelerator to raise a revolution speed of the engine for maintaining the speed, but in this situation the operator concentrate his or her mind on grasping a form of the land to be improved or on operation of a blade, so that the operator can not operate the accelerator smoothly, and therefore engine stop frequently occurs.
Once the engine stop occurs, the vehicle stops once, and also the work for improving the land surface with a blade is stopped once, so that a step caused by operation stop of the blade is generated on the surface of the improved land, which makes it difficult to improve the land surface with the expected precision. If this event occurs, sometimes it is required to restart the work for improving the land surface from the initial stage, which is troublesome. Especially when the engine stop occurs in the finishing stage of the work for improving the land surface, if only the stepped section is again improved, finishing with high precision is impossible, and the work for re-finishing must be performed repeatedly in a wide area around the stepped section, which substantially spoils the work efficiency.
On the other hand, when a large capacity engine capable of generating a large low speed torque even during running at a low speed is used, the engine stop never occurs even when a higher duty is required for the engine, and the work for improving the land surface can be carried out continuously. However, the large capacity engine as described above requires a large space for mounting it on a vehicle, so that also the size of the vehicle is required to be large and also the cost becomes higher. Recently there is the tendency for suppressing the low speed torque to the same level as that for a small capacity engine to satisfy the emission control requirements becoming more and more strict, and there is the need for efficiently preventing occurrence of the engine stop with the engine having the low speed torque as described above.